As a coating method of coating a coating solution on a running substrate, there are known the following two coating methods. One is a post-measuring method in which a coating solution in an amount more than a coating solution amount to give a necessary coating thickness is in advance ejected onto a substrate and the extra coating solution was removed through any coating solution removing means. As the post-measuring methods, there are known a blade coating method, an air knife coating method, a wire bar coating method, a gravure coating method, a reverse coating method and a reverse roller coating method.
The other one is a pre-measuring method in which a coating solution in an amount to give a necessary coating thickness is in advance ejected onto a substrate. As the pre-measuring methods, there are known an extrusion coating method employing a slit-type die coater, a slide coating method employing a slide coater, a curtain coating method, and a coating method employing an ink jet head. In a pre-measuring coating apparatus, a slit-type die coater can meet high coating accuracy, high quality, high coating speed, thin film coating and a multi-layer coating property as compared to other pre-measuring coating apparatuses. Accordingly, the a slit-type die coater is employed to manufacture an optical film, an ink jet recording paper sheet, a recording material for thermal development and an organic electroluminescence element (hereinafter also referred to as organic EL element).
The organic EL element is used in a display field, illumination field and the like, and generally manufactured by a vapor deposition method. In recent years, however, the organic EL element is manufactured by a coating method called a roll-to-roll method in view of productivity or manufacturing cost.
Further, requirement for high functionality, thickness reduction, coating thickness uniformity or high productivity of an organic EL element has increased.
For example, when an organic compound layer (such as a hole transporting layer, a light emission layer, etc.) constituting an organic EL element is coated by a coating method, a viscosity of a coating solution used is not more than 4.0 mPa·s. The hole transporting layer as the organic compound layer has a thickness of from 5 to 500 nm, and one layer constituting the light emission layer has a thickness of from 2 to 100 nm, both layers being thin. Accordingly, uniformity of the organic compound layer thickness has an influence on performance of the organic EL element, and requirement for the thickness uniformity has increased.
In a conventional extrusion coating method employing a slit-type die coater in which a coating solution with a viscosity of several mPa·s is coated at a coating speed of from several tens m/minute to several hundreds m/minute to form a thin film, the coating thickness distribution is reduced to several percents by carrying out coating under such conditions that a pressure upstream of the slit-type die coater is reduced, the coater gap is at least about 100 μm which is from 3 to about 10 times the wet coating thickness, and the coater gap accuracy is several percent (accuracy from the limit of mechanical accuracy such as straightness of the slit-type die coater or the cylindricity or rotation accuracy of the backup roller. Herein, the coater gap implies a clearance between the lip tip of the slit-type die coater and the substrate.
That is, in the conventional extrusion coating method, to maintain the bead under increased pressure is condition of stable coating. The coater gap is preferably larger, since a smaller coater gap produces poor coating thickness distribution. However, when the coater gap is enlarged to two times or more the wet coating thickness, the bead is put under negative pressure atmosphere caused due to accompanying air generating with transport of a substrate, and cannot maintain its shape, which results in incapability of stable coating.
In order to prevent the bead from being put at negative pressure, a method is carried out which cuts the accompanying air by reducing a pressure upstream of the slit-type die coater. When the pressure is not reduced, the coater gap is from 1.5 to 2 times the wet coating thickness, and when the pressure is reduced, the coater gap is at most ten times the wet coating thickness.
Accordingly, when coating is carried out at a wet coating thickness of not more than 10 μm employing an extrusion coating method, it is necessary to lessen the water gap to not more than 100 μm even under reduced pressure. Such a small gap produces poor coating thickness distribution.
A method has been hitherto studied in which employing a low viscosity coating solution, coating is carried out according to an extrusion coating method employing a slit-type die coater to form a stable thin layer. A method is disclosed in for example, Japanese Patent O.P.I. Publication No. 2007-98224 in which when a coating solution with a low viscosity is coated into a thin layer on a continuously running web (corresponding to a belt-shaped substrate in the invention) supported on a backup roller employing a slot die (corresponding to a slit-type die coater in the invention), a slot die with a slot die tip in the concave form, in which a clearance between the backup roller and the center in the width direction of the slot die tip is different by 1 to 10 μm from that between the back roller and each of both edges in the width direction of the slot die tip, is employed in order to prevent wet expansion, wet shrinkage or distortion in the die width direction of the slot die.
As a coating apparatus coating a coating solution with a viscosity of from 1 to 10 mP·s on a substrate (corresponding to the belt-shaped substrate used in the invention), an extrusion coater (corresponding to the slit-type die coater used in the invention) having a coating solution ejecting slit width of from 60 to 120 μm and a manifold diameter of from 12 to 18 mm is disclosed in Japanese Patent O.P.I. Publication No. 2006-305548.
However, it has proved that in the extrusion coating method employing a slit-type die coater disclosed in the above patent document, when a coating solution with a low viscosity is coated, control of wet expansion of the coating solution on coating is insufficient, and when the coating solution is coated at a wet coating thickness of not more than 5 μm, the coating width and thickness cannot be sufficiently controlled, which results in non-uniformity of the thickness.
In order to improve productivity as compared to the roll-to-roll method, a method of manufacturing an organic EL panel has been studied in which an organic compound layer (for example, a hole transporting layer, a light emission layer or the like) is coated to be in accordance with plural rows of electrodes provided on a wide substrate employing a slit-type die coater to form multiple coating layers in the stripe shape, thereby obtaining an organic EL element, and then cut into an individual organic EL element. As the method to form multiple layers in the stripe shape, a method is disclosed in Japanese Patent O.P.I. Publication No. 2001-6663 which employs an extrusion coater having a slit with guides provided therein to divide into many small slit parts. However, it has been found that when employing such an extrusion coater, a coating solution with a low viscosity is coated, control of wet expansion of the coating solution is insufficient, and it is difficult to form multiple stripe-shaped coating films.
Further, a method has been studied in which a coating solution with a low viscosity is coated on a substrate to stably form multiple coating layers in the stripe shape. For example, a method is known in which employing a slit-type die coater with many grooves provided at a specific pitch at the land of the die head, a coating solution is ejected into a film form from the slit-type die coater and coated on a substrate, the slit die coater increasing a coating gap (corresponding to a coater gap) between the die coater and the substrate so that the coating solution ejected into a film is divided by the grooves and coated to form multiple coating layers in the stripe shape separated from each other on the substrate (refer to Patent Document 1).
However, it has been found that the method disclosed in the Patent Document 1 has shortcomings as follows.
1. Since a coating solution is supplied to a slit-type die coater under increased pressure and ejected from the slit outlet thereof, a large amount of the coating solution is ejected, and as a result, coating at a wet coating thickness of not less than 25 μm is possible, but when a coating solution with a low viscosity of not more than 3.0 mPa·s is coated at a low wet coating thickness of from 0.1 to 5.0 μm, the width and thickness of the resulting coating layers in the stripe shape are not stabilized.
2. The coater gap is susceptible to vibration due to transport of a substrate, a straightness of a slit-type die coater, or a cylindricity or vibration due to rotation of a backup roller used as a substrate supporting member, the width of coating portions and that of non-coating portions are influenced by variation of the coater gap and likely to vary. Such a variation of the coating width dimension results in instability of coating in the stripe-form.
3. Since a coating solution is supplied to a slit-type die coater under increased pressure and ejected from the slit outlet thereof, a coating solution with a low viscosity enter the grooves which makes it difficult to divide the ejected coating solution, resulting in incapability of forming coating layers in the stripe shape. Even in the case the coating solution shortage at the groove portions is produced by increasing the coater gap between the substrate and the slit-type die coater to form coating layers in the stripe shape, the coating solution shortage extends to other portions which makes it difficult to control the width of coating portions and non-coating portions and results in incapability of stable coating.
4. Since coating portions and non-coating portions are formed according to the coater gap size, the method can be applied to coating at a small pitch (coating portion width+non-coating portion width), but is low in a degree of freedom of establishment of coating portions and non-coating portions.
In view of the above, a coating method has been desired in which employing an extrusion coating method employing a slit-type die coater, a coating solution with a low viscosity is coated on a substrate, so that non-coating portions and coating portions are alternately formed in the stripe shape in the conveyance direction of the substrate, whereby the coating thickness and width of each coating layer formed in stripe shape are stabilized, and an organic electroluminescence element prepared employing the coating method has been desired.